Aflatoxin B1 (AFB1) is a potent environmental carcinogen produced by the common molds Aspergillus flavus, A. parasiticus, and A. nominus. Human exposure results principally from the ingestion of stored foodstuffs contaminated with these molds. Carcinogenicity is associated with the conversion of AFB1 to its 8,9-oxide by the hepatic cytochrome P450-dependent monooxygenase system. Ingestion of food contaminated with fungal aflatoxins is believed to contribute to the high incidence of hepatoma and chronic liver disease in subtropical regions.
Hayes, J. D. et al. (1993; Cancer Res. 53:3887-3894) reported that expression of an aldehyde reductase (AR) is induced in the liver of rats fed on an ethoxyquin-containing diet. Ethoxyquin activates transcription of several liver detoxification enzymes. Induction of expression of the aldehyde reductase is correlated with resistance to AFB1-induced carcinogenesis. The enzyme, AFB1-AR, catalyzes the formation of a dialcohol from the cytotoxic dialdehyde form of AFB1-8,9-dihydrodiol (Judah, D. J. et al. (1993) Biochem J. 292:13-18). Ellis, E. M. et al. (1993; Proc. Natl. Acad. Sci. USA 90:10350-10354) isolated cDNA which encodes AFB1-AR and observed that the AFB1-AR protein sequence defines a previously unrecognized subclass of the aldehyde reductase superfamily. AFB1-AR is 327 amino acids in length and contains a putative active site histidine at position 109.
Primary hepatocellular carcinoma (HCC) occurs with high frequency in eastern Asia and sub-Saharan Africa. In these areas of the world, chronic hepatitis B virus (HBV) infection is the best described risk factor for HCC; however, only 20-25% of HBV carriers develop HCC (McGlynn, K. A. et al. (1995) Proc. Natl. Acad. Sci. USA 92:2384-2387). In certain areas of the HCC endemic regions, an unusual mutational hot spot has been reported in the p53 tumor suppressor gene. This mutation, at codon 249 in exon 7, is an AGG-to-AGT transversion (arginine to serine). The geographic distribution of these mutations and the similarity of these mutations to the transversions produced in vitro by AFB1 suggest that exposure to AFB1 causes the p53 mutation. Demonstration of the preferential mutability of p53 codon 249 by rat liver microsome-activated AFB1 in HepG2 cells supports this hypothesis (Aguilar, F. et al. (1993) Proc. Natl. Acad. Sci. USA 90:8586-8590).
AFB1 is metabolized and detoxified in the liver via detoxification pathways. McGlynn et al. (supra) proposed that genetic variation in AFB1 detoxification genes may define susceptibility to the effects of AFB. To test their hypothesis, genetic variation in two AFB1 detoxification genes, epoxide hydrolase (EPHX) and glutathione S-transferase M1 (GSTM1), was compared with the presence of serum AFB1-albumin adducts, the presence of hepatocellular carcinoma (HCC), and with p53 codon 249 mutations. Mutant alleles at both loci were significantly over-represented in individuals with serum AFB1-albumin adducts in a cross-sectional study. In a case-control study mutant alleles of EPHX were significantly overrepresented in persons with HCC. The relationship of EPHX to HCC varied by hepatitis B surface antigen status and indicated that a synergistic effect may exist. p53 codon 249 mutations were observed only among HCC patients with one or both high-risk genotypes. These results indicate that individuals with mutant genotypes at EPHX and GSTM1 may be at greater risk of developing AFB1 adducts, p53 mutations, and HCC when exposed to AFB1. Hepatitis B carriers with the high-risk genotypes may bear even greater risk than carriers with low-risk genotypes. These findings by McGlynn et al. (supra) support the existence of genetic susceptibility in humans to the environmental carcinogen AFB1 and indicate that there is a synergistic increase in risk of HCC with the combination of hepatitis B virus infection and susceptible genotype.
The discovery of a new human aflatoxin B1 aldehyde reductase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of gastrointestinal and neoplastic disorders.